Unlike the capilliary wick in a conventional heat pipe, the pumping rate across the membrane, separating the solvent and solution reservoirs, in an osmotic pumped heat pipe, does not automatically match the heat input rate. Thus, the working fluid, comprising the solvent, would continue to flow through the membrane and into the solution reservoir, even after the heat source was removed from the evaporator unless flow were prevented. Conventionally, flow of working fluid is shut off by a mechanical valve, using the effect of pressure retarded osmosis, or by regulating the heater power. However, in practice the heat input cannot always be regulated, and a mechanical valve is complex and expensive to implement within an osmotic pumped heat pipe.